This description relates to two-dimensional and three-dimensional projecting.
Stereoscopic projection, commonly called three-dimensional (3D) projecting, delivers slightly different images to each eye of a viewer, which gives the illusion of depth when the viewer's brain assembles the two images into a single scene.
In a polarization-based 3D projection system, two projectors are used, one for each eye, and polarizing filters are used to polarize the light from each projector orthogonally to the other. The viewer wears glasses with corresponding polarizing filters, so that each eye receives only light projected from the corresponding projector.
In anaglyphic projection, the two images are each color-shifted, one into the red end of the visible spectrum and one into the blue end. The viewer wears glasses with red and blue filters, one for each eye, so that each eye sees only the image shifted into the corresponding color. The viewer's brain reassembles the two images into a single reduced-color image with the illusion of depth. Such a system also works with still images, which can be printed with the two color-shifted images overlaid.
A third approach projects alternating images for each eye, and glasses, for example with LCD shutters, actively block the view of the eye opposite the image currently being projected.